Method for forming anodic oxide film on aluminum or aluminum alloy



Dec. 29, 1970 TAKAsHl suzuKl EI'AL 3,55l,303

METHOD FOR FORMING ANoDIc oxIDE .FILM

oN ALUMINUM oR ALUMINUM ALLoY Filed Aug. 28. 1967 CRCK DEN/ TY I I I I I C C2 03 b G4 0 TH/cK/vs afF/LM ATTORNEYS United States Patent O Int. Cl. C23b 5/50 U.S. Cl. 204-35 1 Claim ABSTRACT OF THE DISCLOSURE A method for forming an anodic oxide film of a desired thickness on an aluminum or an aluminum alloy which has an improved withstand voltage when a bending stress is applied, which comprises forming an anodic oxide film thinner than a desired thickness, cracking said film in a suitable manner, then conducting an anodic oxidation again and, if necessary, repeating said cracking and said anodic oxidation, whereby there is formed an anodic oxide film on an aluminum or an aluminum alloy.

The present invention relates to a method for treating the surfaces of an aluminum and its alloy to obtain an insulating film which has a high bending resistance.

As an anodic oxide film obtained by conducting an anodic oxidation on an aluminum or its alloy is excellent in insulating property, it is used for a surface insulating material on a linear-or strip-type electrical conductor made of aluminum or aluminum alloy (this will be described as a conductor hereinafter). However, an anodic oxide film obtained by the generally known method for anodic oxidation does scarcely show flexibility and is cracked with an elongation of only 0.4-5 That is when a conductor having a surface subjected to anodic oxidation is 'bended with a curvature smaller than a certain value, a tensile stress comes to be applied on the film of outer surface, so that cracks occur running in the direction perpendicular to that of bending. When the conductor is bended with a smaller curvature and with increase of the elongation of the outer surface thereof, more cracks break out additionally and at the same time apertures of previously formed cracks increase. The increase of aperture of crack results in a reduction of the insulating property of the film. Therefore, although the conductor having an anodic oxide film formed by the ordinary method has advantages that the thermal durability and the adhesive property of the film are excellent, it has a serious disadvantage that the withstand voltage of the film becomes lower when the conductor is bended with a curvature not larger than about 20 times as large as its diameter or thickness, so that with a smaller curvature than said value it can not be substantially employed. There have been proposed heretofore some methods for improving the fiexibility of film by varying electrolytes or electrolitic conditions for the anodic oxidation. However, these methods were not able to largely improve the resistance to bending.

As one of methods for improving the property of the conductor against bending, on which an anodic oxide film is formed, there is considered a method wherein a crack density (the number of cracks per unit length in a bending direction breaking out in the outer surface of a conductor) is increased with the same film thickness and the same bending curvature. The present invention has been accomplished on the basis of this consideration and "ice the experimental result that with the same quality of film, the crack density is reduced with the same elongation rate, as the film thickness is increased.

According to the present invention, there is provided a method for forming an anodic oxide film, whose bending resistance is improved, on an alurninum and an alurninum alloy.

According to the present invention, there is provided a method for forming an anodic oxide film, whose insulating property at the time of applying a tensile stress is improved, on an aluminum or an alurninum alloy, characterized by forming an anodic oxide film having a thickness thinner than a desired value on the surface of an aluminum or an aluminum alloy, then elongating said anodic oxide film so as to break out cracks over all region of said' film, then again conductng an anodic oxidation on said aluminum or aluminum alloy so as to increase the thickness of said whole anodic oxide film and, if necessary, repeating the above Operations, whereby an anodic oxide film of a desired thickness is formed.

The present invention will be explaned in detail further with reference to the drawings.

In the attached drawings, FIG. 1a shows schematically a state of crack formation in an anodic oxide film prepared in accordance with the present invention and FIG. 1b shows schematically a state of crack formation in an anodic oxide film formed by the conventional method. FIG. 2 shows graphically the results of experiment regarding the relation between the thickness of anodic oxide film and the density of cracks (the number of cracks per unit length in elongation direction) formed in the oxide film at the time of giving a predetermined elongation.

`Referring to FIG. l, 1 is conductor, 2 and 2' are anodic oxide films and 3 indicates cracks formed in the outer anodic film 2. d1 and dz mean sizes of the crack apertures respectively, where d1 d2. That is, dl in the case that a density of cracks formed in the film 2 is larger (FIG. la) is smaller than dz in the case of a smaller density (FIG. 1b). When the conductor is bended to elongate the film, the higher the density of cracks formed in the film becomes, the smaller the aperture of cracks becomes and the reduction of the withstand voltage of the film, which is caused by the crack formation, also becomes smaller. This is the consideration, on which the present invention is based.

FIG. 2 shows the relation between the crack density and the thickness of film obtained from an experiment. This relation was obtained with a given quality and Shape of aluminum specimens, a given anodic oxidation condition and a given elongation rate of the film. The Variation of the thickness of the film was efiected by changing time or voltage for anodic oxidation.

When an anodic oxide film having a thickness of a is to be formed on a conductor, such a thickness should not be attained at a time, but at first a film having a thickness of b (where b a) should be formed. Then the film is elongated to produce cracks over all region of the film. After that, again an anodic oxidation is conducted and stopped when the thickness of film comes to be a. It has been found that the film formed in such a manner has a tendency that when elongated, cracks break out at the same points where the cracks were formed previously, and that density of the newly formed cracks is almost equal to that of cracks in the case of the film thickness of b.

As is clear from the relation in FIG. 2, in order to increase the density of cracks, it is preferable to make the film thickness b as small as possible. If a difference between the film thickness b and the final film thickness a is too large, however, after forming the film subsequently up to the thickness of a, cracks break out by elongating the film at points independent of those, where cracks Were formed at the time of the film thickness of b, only in the same state as in the case a film of a in thickness is formed from the beginning at a setting. In other words, the density of cracks does not become m but l. Therefore, there is a limitation in a ratio of the film thickness of b to that of a. From the results of experiments carried out by the inventors, it has been made clear that when the film thickness of a becomes above times as large as the film thickness of b, the aforesaid tendency comes to appear. In order to make the density of cracks as large as tion four times under the conditions shown in Table 1 in accordance with the present invention. The density of cracks, which were formed in the direction of bending when each sample was bended along a rod of a Constant curvature in air, and the dielectric breakdown voltage between each sample and a metal foil when each sample was bended with a defined curvature in oil with contacting the metal foil electrode on its outer surface were measured. The results are shown in Table 2. Further, when the sample B was prepared, the crack formation was effected by continuously rolling the sample on steel rollers of 5 mm. in diameter having a smooth surface.

TABLE 1.-CONDITIONS FOR PREPARING SAMPLE B Film thickness, ;z

possible and moreover to increase the thickness of film, it is necessary to repeat a process comprising anodic oxidation and crack formation.

Referring to FIG. 2, in order to obtain a film of a final thickness of a, firstly a film of cl in thickness is formed and cracked. The crack density at this time is designated as nl. Then an anodic oxidation is conducted again to make the fihn thickness C2 of about 5 times as large as cl, followed by breaking out cracks again. When cracks are formed in this film, the density of cracks does not become n2, but nl. Repeating further and further this process, the thickness of the film is increased to C3, c., with reaching finally a. The thus produced film shows, in spite of the thickness of a, a density of cracks formed by its elongation almost equal to that in the case of the film thickness of C1, that is, nl.

As electrolytes, liquids such as sulfuric acid, oxalic acid, phosphoric acid, chromic acid and sulfamic acid forming a porous film are suitable. Further, liquids such as boric acid and ammonium borate may be employed. In this case increasing of the film thickness is eifected by increasing a forming voltage.

As methods for breaking out cracks, that is, methods for elongating a film, there are a method wherein the conductor, on the surface of which a film is formed is rolled on rolls of a suitable diameter having smooth surfaces or having ribbed surfaces and a method wherein a rapid temperature change is applied to the conductor having a film formed by using a difference between the thermal expansion coefiicient of aluminum or aluminum alloy and that of anodic oxide film (for example, at a temperature range of -30, C., the thermal expansion coeflicient of aluminum is 24.5 10-6/ C., while that of the film is 4.5 10 6/ C).

Crack formation may be carried out in air after taking out a conductor from a electrolytic bath and washing the same with water, but it can be also conducted in the electrolytic bath. In the latter, only one of the electrolytic bath is required and water-washing is not necessary more than once after all the anodic oxidations are finished.

Example An anodic oxidation was conducted on an aluminum foil of 100p. in thickness and 85 mm. in width having an aluminum purity of 99.4% in a 10% sulfuric acid solution at a current density of 0.8 a./dm.2 to form a film on the surface. The final film thickness was made to 10a. There were produced the sample A, in which a film of 10a in thickness was formed at a time and the sample B, in which a film of 10a in thickness was formed by re- Peating a process ef anodic oxidaton and crack forma- TABLE 2.-RESULTS OF BENDING-RESISTANCE TESTS As is clear from Table 2, the anodic oxide film prepared in accordance with the present invention shows an extremely smaller reduction of its insulating characteristic at the time of application of a tensile stress comparing with that in accordance with the conventional process. Moreover, the method of the present invention can be easily carried out with an ordinary apparatus for anodic oxidation and, therefore, has an extremely large advantage in the industry.

What is claimed is:

1. A method for forming an anodically oxidized film on the surface of aluminum or an alloy thereof, comprising the steps of elongating an anodically oxidized film formed on the surface of aluminum or an alloy thereof to break out cracks over the whole regions of said film, and then anodically oxidizing the resultant film to increase the film thickness to break out further cracks when the film is again elongated, said increased film thickness being in the range of less than 10 times the thickness of the initially formed film and repeating said process a plurality of times to form a film having a density of cracks of more than 60 cracks/ mm. broken out in the direction of elongation.

References Cited UNITED STATES PATENTS 2,578,400 12/1951 Cohn 204-42 2,739,931 3/ 1956 Bernstiel 204-35 2,788,317 4/1957 Sonnino 204-35 2,912,369 11/1959 Altenpohl et al. 204-58X 2,930,739 3/1960 Burnham 204-58X 2,951,025 8/1960 Mostouych et al 204-58X 2,993,848 7/1961 Brennan 204-5EX 2,995,502 8/1961 Ramirez et al 204-58X 3,074,857 1/1963 Altenpohl 204-58X 1,853,437 4/1932 Kuttner 204-58X 1,904,418 4/1933 Dantsizen 204-58X JOHN H. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner US. C1. .X..R. 204-42. 

